1. Field of the Invention
The present invention relates to a controller for a thermal ink jet printer. Specifically, the present invention is a controller for controlling the spot size associated with an ink jet printhead which responds to the temperature of the ink in the printhead.
2. Description of Related Art
In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors of a printhead to create a desired image on a image receiving member. The printhead typically comprises an array of ejectors for conveying ink to the image receiving member. The printhead may move back and forth relative to the image receiving member in order to print the image, or the array may extend across the entire width of the image receiving member. In either case, the image receiving member moves perpendicularly relative to the linear array of the printhead. The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the image receiving member. One patent using the general configuration of a typical ink jet printhead is, for example, U.S. Pat. No. 4,774,530 to Hawkins.
When a quantity of ink, in the form of a droplet, is ejected from the ejector to the image receiving member, the resulting spot of ink becomes part of the desired image. Uniformity in the spot size of a large number of droplets is crucial to maintaining image quality in ink jet printing. The human eye is very sensitive to changes in spot size, especially when shaded areas and graphics are being produced. If the volume of droplets ejected from the printhead over the course of producing a single image is permitted to vary widely, this lack of uniformity in droplet volume will have noticeable effects on the ink spot size of the image, and therefore on the quality of the image. Similarly, if volumes of droplets ejected from the printhead differ during subsequent printings of the same image, then printing stability cannot be maintained; this is particularly important in color printing, where the colors produced are highly dependent on the volume ratios of the ejected drops which combine to produce the desired colors.
The most common and important cause of variance in the volume of droplets ejected from the printhead is variations in the temperature in the printhead over the course of use. The temperature of the ink, before vaporization by the heating element, substantially effects the viscosity of the ink. Control of the temperature of the printhead then has long been of primary concern in the art.
In order to maintain a constant spot size from the ink jet printhead, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., assigned to the assignee of the present application. In this patent, the printhead has integrated into it a number of heater resistors and a temperature sensor which operate to heat the printhead to an optimum operating temperature and maintain that temperature regardless of local temperature variations.
U.S. Pat. No. 4,791,435 to Smith et al. discloses an ink jet system wherein the temperature of the printhead is maintained by using the heating elements of the printhead not only for ejection of ink but for maintaining the temperature as well. The printhead temperature is compared to thermal models of the printhead to provide information for controlling the printhead temperature. At low temperature, low energy pulses are sent to each channel, or nozzle, below the voltage threshold which would cause a drop of ink to be ejected. Alternatively, the printhead is warmed by firing some droplets of ink into an external chamber instead of onto the image receiving member.
PCT Application No. U.S./90/10541 describes a printhead in which the heating cycle for the ink is divided into several partial cycles, only the last of which initiates bubble formation and ejection of a droplet. In this printhead, therefore, the liquid ink is first preheated to its preselected temperature, the ink having known volume and viscosity characteristics, so that the behavior of the ink will be predictable at the time of firing.
PCT Application No. U.S./90/10540 discloses a printhead control system wherein the temperature of the liquid ink is compared with a predetermined threshold value, and if it exceeds this threshold value the pulse energy (proportional to the square of the voltage to the heating element times duration of the pulse) is reduced. According to this patent, the pulse energy may be varied by controlling either the voltage, the pulse duration, or both.
U.S. Pat. No. 4,736,089 to Hair et al. discloses a thermal printhead (as opposed to an ink jet printhead) wherein printhead temperature is sensed by a voltage generating diode on the printhead itself. A detected temperature of the printhead is used to establish a preselected reference level. Bi-stable means are coupled to the thermal printhead to print or not print in a given time. Control means are used to turn the bi-stable means on when the control voltage is less than the reference level related to the temperature, and turns the bi-stable means off when the control voltage exceeds the preselected reference level, thus causing the time duration of a voltage pulse to the thermal printing means to be dependent on temperature.
U.S. Pat. No. 4,980,702 to Kneezel discloses a thermal ink jet printhead wherein outputs from a temperature sensor in the printhead are compared to a high or low level temperature reference. If the sensed printhead temperature is below the reference level, power to the heater in the printhead is turned on. If the temperature sensed is too high, the heater is turned off. The print-head is configured so that the temperature sensor and the heater in the printhead are in close proximity.